The present invention relates to a method of producing a polycrystalline thin film of MgO which has good crystal orientation, a low surface roughness and a flat surface, and which can be used as an intermediate layer of a superconductor, an insulation layer of a plasma display panel or the like.
Recently, technology for forming various films of good orientation on a substrate made of a polycrystalline or amorphous material by means of a sputtering apparatus has been employed. The technology is applied, for example, to the fields of oxide superconductors, optical thin films, magneto-optical disks, interconnection substrates, high-frequency filters and cavity resonators, in all of which there is the problem of forming a polycrystalline thin film having a stable film quality and good crystal orientation on the substrate. This is because a polycrystalline thin film having good crystal orientation makes it possible to improve the quality of a superconducting thin film, an optical thin film, a magnetic thin film or an interconnection thin film to be formed thereon.
One typical example of the polycrystalline thin film formed on a substrate is a polycrystalline thin film of MgO.
A method of producing the polycrystalline thin film of MgO in the prior art is the sputtering method in which particles of a target material are dislodged and then deposited on the substrate. The sputtering method can be implemented in the form of diode glow discharge sputtering which uses glow discharge across a pair of electrodes, magnetron sputtering or other processes.
In such sputtering processes, a target made of the same material as the polycrystalline thin film to be formed is bombarded by accelerated particles such as ions in a vacuum of about 10xe2x88x921 Pa, so that the particles of the target material are dislodged and then deposited on the substrate.
The accelerated particles such as ions can also be made to collide with the target by the use of glow discharge.
However, with these methods of producing the polycrystalline thin film of MgO, the substrate must be held at a high temperature in a range of from 300 to 1000xc2x0 C., in order to form a polycrystalline thin film of good crystal orientation, in which planes of low indices such as (100) and (110) are oriented vertically, on a substrate made of a polycrystalline or amorphous material such as a metal of non-aligned crystal orientation.
If an MgO film is formed at such a high temperature, the MgO crystal grains become coarse when depositing the MgO particles on the substrate, thus leading to the problem that the surface roughness of the polycrystalline thin film increases, resulting in a rough surface. When the polycrystalline thin film has a rough surface, the quality of the thin film formed on the polycrystalline thin film becomes unsatisfactory, that is, the crystal orientation is disturbed.
Also with these production methods, since the ions which dislodge the particles from the target are generated by using glow discharge in the vessel which is used to form the film, a plasma is generated in the vacuum vessel and the substrate is placed in the plasma. Therefore, the crystal orientation of the MgO is expected to be adversely affected by the plasma which hits the substrate surface.
For these reasons, a thin film of good crystal orientation cannot be obtained when a polycrystalline thin film of MgO is formed as an intermediate layer on a substrate of a polycrystalline or amorphous material and another thin film is formed on this intermediate layer.
For example, when a polycrystalline thin film of MgO is formed as an intermediate layer on a substrate such as a metal tape by the sputtering method and an oxide superconductor is made by forming an oxide superconducting layer on the intermediate layer by a laser beam deposition or sputtering process, the oxide superconducting layer thus formed shows a critical current density lower than that of an oxide superconducting layer formed on a single crystal substrate.
Such a problem suggests a poor crystal orientation of the oxide superconducting layer. Since the oxide superconducting layer is grown so as to match the crystal structure of the intermediate layer when forming the film, the crystal orientation of the oxide superconducting layer is affected by the conditions of the intermediate layer, namely the crystal orientation and the surface conditions thereof. Thus the problem is assumed to be caused by poor crystallinity of the intermediate layer, namely the polycrystalline thin film of MgO, and/or the rough surface thereof.
In the production methods of the prior art, as described above, it has been difficult to form a polycrystalline thin film of MgO of good crystal orientation and satisfactory surface roughness on a polycrystalline or amorphous substrate.
The present invention has been made to solve the problems described above and an object of the present invention is to provide a method of producing a polycrystalline thin film of MgO which has good crystal orientation, low surface roughness and a flat surface.
In order to solve the problem described above, the present invention provides a method of producing a polycrystalline thin film of MgO, wherein an ion beam sputtering process is employed to form the polycrystalline thin film of MgO on a substrate by irradiating a target with an ion beam to dislodge particles from the target and deposit the particles on the substrate.
With the method of producing the polycrystalline thin film of MgO of the present invention, since an ion beam sputtering method is employed wherein the target is irradiated with an ion beam to dislodge particles from the target and the particles are deposited on a substrate, thereby forming the polycrystalline thin film of MgO, the substrate is not heated to a high temperature nor exposed to plasma, and therefore it becomes possible to form a polycrystalline thin film of MgO which has good crystal orientation, a lower surface roughness and a flat surface.
With the method of producing the polycrystalline thin film of MgO of the present invention having the constitution described above, it is preferable to form the film by the ion beam sputtering process in an atmosphere with a pressure reduced to 3.0xc3x9710xe2x88x922 Pa or lower, because this makes it possible to improve the sputtering rate, form a polycrystalline thin film of uniform MgO crystal grains on the substrate and decrease the surface roughness.
Also, because the film is formed in a vessel (film forming chamber) which is evacuated to 3.0xc3x9710xe2x88x922 Pa or lower, a higher degree of vacuum than that used in conventional sputtering processes (1.0xc3x9710xe2x88x921 Pa), the crystallinity of the MgO can be improved, namely the (110) plane can be oriented vertically, and a polycrystalline thin film of MgO of better crystal orientation can be formed.
With the method of producing the polycrystalline thin film of MgO of the present invention having the constitution described above, it is preferable to form the film by the ion beam sputtering method while keeping the substrate temperature at 300xc2x0 C. or lower, because this results in a polycrystalline thin film of MgO having a surface roughness of about 10 nm or less, thus forming a polycrystalline thin film having satisfactory surface conditions.
Also, with the method of producing the polycrystalline thin film of MgO of the present invention having the constitution described above, it is preferable to form the film by the ion beam sputtering process while keeping the substrate temperature in a range from 0 to 200xc2x0 C., because this results in a polycrystalline thin film of MgO having a surface roughness in a range from about 2 to 3 nm, thus forming a polycrystalline thin film having satisfactory surface conditions.
With the method of producing the polycrystalline thin film of MgO of the present invention having the constitution described above, it is preferable to form the film by the ion beam sputtering process with the ion beam energy being controlled in a range from 1000 to 2000 eV, because this allows a sufficient amount of energy to be imparted to the Mg and O atoms which have been dislodged from the target, thereby promoting migration to the MgO film surface and achieving favorable surface conditions.
With the method of producing the polycrystalline thin film of MgO of the present invention having the constitution described above, it is more preferable to form the film by the ion beam sputtering process with the ion beam energy controlled in a range from 1200 to 1500 eV, because this allows a sufficient amount of energy to be imparted to the Mg and O atoms which have been dislodged from the target, thereby promoting migration to the MgO film surface and achieving favorable surface conditions, while avoiding the adverse effect of an increase in the target temperature.
With the method of producing the polycrystalline thin film of MgO of the present invention, as described above, the sputtering rate can be improved to achieve a better film quality and obtain a polycrystalline thin film of MgO of further improved crystal orientation, by carrying out the ion beam sputtering process in an atmosphere of a reduced pressure of 3.0xc3x9710xe2x88x922 Pa or lower and/or at a substrate temperature of 300xc2x0 C. or lower.